One premise underlying this proposal is that fibrin formation is an irreversible process; as a result, the structure of fibrin is determined by the kinetics of a self-assembly process. In order to understand the assembly, we must determine the nature of the intermediates, the rates at which the intermediates and product are formed, and the equilibria between these species. This will be done by observing the assembly at low concentration (where it is slow) and applying relatively mild changes in conditions that are known to greatly affect the kinetics and equilibria of assembly but do not cause protein denaturation. We plan to work simultaneously towards a better description of the assembly process and the structure of fibrin. Some important structural details are: the conformation of monomeric fibrin, the overall shape of the molecule in solution and in the fiber, the packing of molecules in the fiber (spacings and symmetry), the diameter of the fibers, the fiber length between network branchpoints and the structure at the branchpoints. We shall use light scattering in order to follow the formation of fibers and to measure fiber thickness, electron microscopy to determine the size and shape of fibers at different stages of the assembly and to study the frequency and structure of network branchpoints, elasticity measurements to detect and follow the formation of the network of fibers. We shall investigate methods for preparing fibrin in a form suitable for x-ray diffraction studies. We shall devote a major part of our effort to study physiologically important interactions of fibrin with other plasma components using these physical techniques, following at the same time changes in the chemistry of the fibrin with biochemical analytical techniques. For example we shall use SDS gel electrophoresis to detect changes in the covalent structure due to chain scission or crosslinking. Blood components whose interactions with fibrin we shall study include: plasmin, Factor XIII, fibrin degradation products, calcium ion, and thrombosthenin. We shall also study the effects of ancrod, reptilase and crotalase (thrombin-like enzymes from snake venoms) and of ethanol and dextran on the formation of fibrin fibers.